The importance of peers in adolescent sexual initiation in impoverished communities has been well documented; what remains elusive are the mechanisms by which peers are influential. The purpose of this proposed prospective study of early sexual initiation is to determine how popular students, in particular, influence the behavior of their peers during the sixth grade school year. Using the Philadelphia Peer Influence Project (PPIP), a prospective study of 1434 sixth graders' risk imitation behaviors at the beginning and end of the school year, we will test three hypotheses of diffusion theory that may explain how popular students influence their peers to initiate sexual intercourse. Hypothesis 1: High-risk popular students directly (i.e. by having a close association such as friendship) increase the risk that peers will initiate early sexual intercourse. Two specific aims will address this hypothesis. Aim la: To create an index of an individual's direct association with a high-risk (sexually experienced) popular student. Aim 1b: To distinguish, using logistic regression, students who initiate sexual intercourse between time 1 and time 2 from those who do not, as a function of their direct association with a high-risk popular student. Hypothesis 2: High-risk popular students indirectly (i.e. by supporting high-risk sexual norms in a school) increase the risk that peers, even those with whom they are not closely associated, will initiate sexual intercourse. Two specific aims will address this hypothesis. Aim 2a: To evaluate if high-risk popular students increase peers' perceptions of high- risk sexual norms within schools between time 1 and 2. Aim 2b: To distinguish, using multilevel hierarchical models, students who initiate sexual intercourse between time 1 and time 2 from those who do not, as function of between-school variation in (1) norms and (2) proportion of high- risk popular students. Hypothesis 3: Popular students both directly and indirectly increase the risk that peers will initiate early sexual intercourse. One specific aim will address this hypothesis. Aim 3a: To model, using multilineal hierarchical models, direct and indirect associations with light-risk popular students. Interventions that are informed by this research will be sensitive to natural currents of influence in adolescent culture; and will start to teach us how to use these natural flows of influence to promote safer behaviors. Prevention efforts resulting from this research would target a few influential students in an effort to reach all students.